Optimize search execution for suggest-only requests

We skip context preprocessing and only execute
suggest phase for suggest-only requests

core/src/main/java/org/elasticsearch/search/builder/SearchSourceBuilder.java

@@ -729,6 +729,14 @@ public final class SearchSourceBuilder extends ToXContentToBytes implements Writ
         return ext;
     }
 
+    /**
+     * @return true if the source only has suggest
+     */
+    public boolean hasOnlySuggest() {
+        return suggestBuilder != null
+            && queryBuilder == null && aggregations == null;
+    }
+
     /**
      * Rewrites this search source builder into its primitive form. e.g. by
      * rewriting the QueryBuilder. If the builder did not change the identity

core/src/main/java/org/elasticsearch/search/internal/DefaultSearchContext.java

@@ -191,6 +191,9 @@ public class DefaultSearchContext extends SearchContext {
      */
     @Override
     public void preProcess() {
+        if (hasOnlySuggest() ) {
+            return;
+        }
         if (scrollContext == null) {
             long from = from() == -1 ? 0 : from();
             long size = size() == -1 ? 10 : size();

core/src/main/java/org/elasticsearch/search/internal/SearchContext.java

@@ -353,6 +353,14 @@ public abstract class SearchContext implements Releasable {
         }
     }
 
+    /**
+     * @return true if the request contains only suggest
+     */
+    public final boolean hasOnlySuggest() {
+        return request().source() != null
+            && request().source().hasOnlySuggest();
+    }
+
     /**
      * Looks up the given field, but does not restrict to fields in the types set on this context.
      */

core/src/main/java/org/elasticsearch/search/query/QueryPhase.java

@@ -116,6 +116,12 @@ public class QueryPhase implements SearchPhase {
 
     @Override
     public void execute(SearchContext searchContext) throws QueryPhaseExecutionException {
+        if (searchContext.hasOnlySuggest()) {
+            suggestPhase.execute(searchContext);
+            // TODO: fix this once we can fetch docs for suggestions
+            searchContext.queryResult().topDocs(new TopDocs(0, Lucene.EMPTY_SCORE_DOCS, 0));
+            return;
+        }
         // Pre-process aggregations as late as possible. In the case of a DFS_Q_T_F
         // request, preProcess is called on the DFS phase phase, this is why we pre-process them
         // here to make sure it happens during the QUERY phase